For example, in vehicles utilizing electric power as propulsion energy, usually, a high-voltage (e.g., 200 V) DC power supply is insulated from a vehicle body to be used as a floating power supply. Ground faults with respect to a ground potential portion of such a floating power supply include a DC ground fault occurring in a DC circuit portion including a DC power supply and an AC ground fault occurring in an AC circuit portion including an AC motor.
In detecting the ground faults and the insulating state, the value of a ground fault resistance, which is obtained from a charge voltage of a flying capacitor charged by a charge circuit including a DC power supply and the ground fault resistance, is used as a criterion for determining whether the state is good or bad. Then, in the case of an AC ground fault, the flying capacitor is charged only during an ON-duty period of an inverter circuit, and thus even if a time period in which the flying capacitor is connected to a charge circuit is the same, the charge voltage of the flying capacitor differs from the case of a DC ground fault in which the flying capacitor is continuously charged. Then, with regard to the AC ground fault, a proposal to obtain a ground fault resistance in consideration of the ON-duty period has also already been made (e.g., Patent Document 1).